High-frequency electrical communication system



Sept. 3; 1946. P. K; CHATTERJEA ET A1. 2.4065803 HIGH FREQUENCYELECTRICAL COMMUNICATION SYSTEM Filed nec. 1o. 1942 AAAA . AttorneyPatented Sept. 3, 1946 HIGH-FREQUENCY ELECTRICAL COMMUNICATION. SYSTEM'Prafulla- Kumar Chatterjea and Leslie Wilfred Houghton', London W. C. 2,England, assignors to Standard Telephones and Cables Limited,

London, England, a. British company v Y Appiication December 10, 1eraseriali No.r4ss,572

- In Great` Britain December 16, 1941 .1 g In nearly all time modulatedpulse systems used for transmitting intelligence of any kind, the'amplitude of the*A signal` obtained at the receiver is some-.function ofthe total time occurring between the lead-ing and trailing edges of avpulse in a trai-n of time Vmodulated pulses and it`V is necessary todetermine in general the duration of each individual pulse toreconstructY the intelligence..` A method of and arrangements forutilising pulseswhich are time modulated as a function of the amplitudeof a sound or like Wave have been proposed in which the desiredintelligence can be obtained from` one edge only of the sai timemodulated pulse'.` K l g The operation of this latter systemv isdependent upon transmitting atrai-nof pulses so time` modulated thatsaid pulses-have a function characteristic of the Aanfipl-itude ofasound: or like Wave (hereinafter also called intelligence Wave), saidtime modulation7 bei-ng characterise'd in that the ,time intervalbetween the limitsof modulation is equal to the time taken for afraction (preferably less than lyg) of a Wavelength period of a radiofrequency (this frequency itself islarge compared with the pulserepetition frequency). At areceiver said pulses .are used 13 claims.(Cl. 25o-'17) by modifying the slope of one of tlie edges ofthepulses-Lpreferably the edge which normally oc-v curs at equal intervalsof time, and preferably the A.trailing edge.- Here thenV the' slopesf ofthe trailing edge are characteristic of' the' amplitudes' of a sound orlike vvave.- Inpractice, itis foundY that actually both edges are'modified.

The special advantages obtained from' these method-syand-arrangementsare various, andpar-j tointerrupt a train' o'foscillations (thesefbeing v at the carrier, i. e. at the said radiofrequency) and thel amplitude ci" the intelligence vwave r'eceived is afunction of the amplitude ofv the damped oscillations at theinterruptions' of the pulse' train, which in turn is dependent at any' ym'oment on tlie time duration of the pulses-tinsV duration beingmodulated byY the intelligence Waveiny the manner describedhereinbefore'.

The main requirement of such a system is1 to have at the transmitter aconstant carrier (or radio frequency generator which'. is set intooscillation b'y one` edge of a sharpsh'ort pulse (say the loading edge).This oscillation buildsrup to a steady value in ati-ineI depending onthesharp'- ticular usefulness lies -inf the dissemination of propaganda,stoppage of unwanted transmissiom or transmission ofintelli'gfence overa large band of frequencies. For example', one sig-nal can transmitintelligence within' i6 dbs: level` over a band Width of 300 kc./s., thesignals being received at frequency intervals throughout this band and'the intervals being. solely governed by the frequency of repetition oftheV pulses. r-

The present invention has for its object to provide another .rr'iethodIof and other arrangements for achieving similar results. v Another.objectof` `the invention is to render frequency modulation intelligiblein an amplitudeV modulation receiver. An electrical signal transmissionsystem according t'o the invention is characterised in that-once a pulsetrain-has been initiatedi. e. a pulse duration of a certainrepetitionfreq'ue'ncy has been rdecidedV upon,v the frequency or phaseof the carrier alone is varied either directly or indirectly accordingto the instantaneous amplitude of` the intelligence Wave.`

nessv of the initial rise' of the' pulse, among other factors, and theduration of the pulse. The'trailing edge of the same pulse in Ythis caseinitiates another forced oscillation intol the circuit', the^v initialamplituilel of this' second sety of oscillations being determi-nedmainly by the'v duration of the pulse, other factors being' constant.

Id'eallyfo'f courseY the edges oi the pulse have to' be absolutelystraight tomake possible the proper control of the initial' stage" ofthe" damped' oscillation, this is inv generall rather di-iicult1 t0achieve without extra precautionin thev circuit design.

If fc be the carrier frequency at which the transmitter output is tuned,a change ofv this j@ equal to l -Afc is-made characteristic of theamplitude ofthe intelligence Wave,` whilst the pulse duration, the pulserepetition frequency, and the pulse slope arel maintained constant. v

According to a feature of the invention the trains ofr oscillations haveinitial amplitudeswhich are characteristic ofthe modulating intelligenceWave. As in all pulse transmission systems the transmission assumes thecharactersticsl of anumber of satellite side bandtransmissions; y

, Using only the1 frequency modulation system faccording'to the" presentinvention, intelligible rece'ption` cannot-nodi be obtained in'receivers de'- signe'df only" for4 amplitude modulated' Waves'v be=cause a detectcr'for' amplitude modulated Waves by itself is immune toVariations of frequency of in its simplest form can be shown to resultin frequency modulation.

In Fig. 1 of the accompanying drawing is shown a vector OP representingan unmodulated carrier frequency w/21r where w=21rf, f being thefrequency. Let its phase be advanced and retarded sinusoidally (say)between -I-oi and p2 with respect to the unmodulated state (e020). the Yvector remaining constant during the operation. This results in apeak-to-peak modulation (p14-qm) but it is important to note that onlyat the extreme excursion positions is the frequency of the rwaveidentical to the unmodulated state, the frequency having alternatelyincreased and decreased With frequency maxima and minima occurring at P,the unmodulated state. In other words, the instants of maximum phase.displacement and maximum frequency change are displaced from each otherby 90 at the modulating frequency. Thus if phase modulation is assumedto be a certain function if, the frequency modulation is inherently therate of change of phase, i. e., the frequency modulation is a function,\I' where \lf is the first differential Vwith respect to time of In Avery simple case is shown here to express the similarity Ybetweenfrequency modulation and phase modulation. It is known that constantphase modulation at all modulating frequencies results actually in morefrequency modulation when higher modulation frequencies are used thanfor lower modulating frequencies.

the applied oscillation amplitude results. This is shown in chain-linein Fig. 3. This would also be the case if three or other odd number ofhalf-cycles of the applied oscillation occurred during the pulseduration. It will be observed that the amplitude of the damped trainsset up in the circuit is dependent upon the frequency of the appliedoscillations, providing the applied oscillations are interruptedcontemporaneously with the trailing edge of the pulse, and that theapplied oscillation Vswings through Zero during the leading edge of thepulse. There thus occurs the interaction of the Adamped waves due to thecontinued damped oscillation of the circuit on account of the appliedoscillation (now terfminated) and the damped oscillation set up bythechange of voltage due to the trailing edge of the pulse. The resultis a damped train of waves whose frequency is that of the appliedoscillations and whose amplitude depends upon the Yrelation between thepulse duration and the periodic time of the applied oscillations.

Thus if the frequency of the applied oscillations is made to depend uponthe instantaneous amplitude of an intelligence or other wave, both thefrequency of the damped waves and the amplitude Vthereof carry theintelligence and the amplitude modulation can be received by receiversdesigned for amplitude modulated waves, whilst frequency modulation canonly be received by receivers designed for reception of frequencymodulated waves. It will be understood by those versed in the art thatthe amplitude of the 4 damped waves generated could be amplified and 'Inone way of carrying out the invention constant duration pulses are used,since the deviation of the carrier frequency is made characteristic ofthe intelligence. This is achieved by frequencymodulating a carrier wavein a certain predetermined manner, and then modulating by a train4 ofpulses of constant duration. The application of phasemodulation toprovide a similar result will be apparent to those skilled in the artfrom the description hereinbefore given on `the similarity between phaseand frequency modulation. The term angular modulation is used herein todenote both frequency and phase modulation.

The invention will be further elucidated in the following description.Referring to Fig. 2 of the accompanyingdrawing, a pulse which is appliedto a resonant circuit is shown as of such duration that one cycle of acarrier wave applied to the same circuit is allowed to occur. The pulseis shown as occurring as the oscillation swings in the positivedirection and the forced oscillation set up by the trailing edge of thepulse is in opposition t0 the applied oscillation. Assuming that theamplitude of the forced oscillation is equal to the amplitude of theapplied oscillation, the resultant amplitude of the circuit oscillationis zero, so that if the applied oscillations are interrupted atthe'moment of the trailing edge of the pulse, the circuit ceases tooscillate. If the oscila lation frequency is as shown in Fig. 3, thevoltage induced by the trailing edge 2 will be in phase with the appliedoscillation, and a voltage of twice passed through a limiting device,hence eliminating the amplitude modulation. When the amplitudemodulation is used, forced reception in receivers for amplitudemodulated waves is obtained over a wide range of carrier frequencies andthese carrier frequencies are determined by the pulse repetitionfrequency of the constant duration pulses applied to the circuit whichproduces the damped trains.

Fig. 4 shows in block diagram an arrangement for carrying out theinvention. Here 3 represents a pulse generator, the output of whichcauses os cillator l!A to start and stop as described, The frequency atwhich this oscillator oscillates at any time is characteristic of thesignal (i. e. the output of the modulation amplifier 5), thig frequencyY being generated by any known method,

The details 'of one circuit arrangement embodying the invention is shownin Fig. 5. InV this arrangement the carrier is provided by an oscillatorcut in and out of use under the control of `that a positive pulse isapplied to the grid thereof from a valve i which is the output valve ofa pulse generator. The frequency at which valve 6 oscillates isgoverned, not only by C, but by the input capacitance of valve 9 whichis in parallel with the tuned circuit l. By means of the circuitassociated with valve 9 the input capacitance of valve 9 can be'made tovary by varying its bias voltage, which can be made to represent a givenmodulationA signal,V obtained from valve if! and its associated circuit.rThe valve il! may, for instance be an amplifier for the intelligencewave to be transmitted, and whose output is passed Vassiduo?,

Y throughV a bias resistance in the grid-cathode circuit of the valve9,. A capacity Il is connected between the grid of 9 and the tunedcircuit LC of valve 6.

The resulting varying dampedl train formed in circuit 'I may be obtainedby any known method of coupling shown in the drawing as' beinginductively coupled to L, .and should .preferably be transferreddirectto a transmitting aerial indicated at A.

What is claimed is:

1. An electrical wave signal transmission system comprising means forgenerating short trains of damped carrier waves, an intelligence wavesource, means for subjecting the carrier waves comprising said wavetrains to angular modulation in accordance with the instantaneousamplitude of the intelligence Wave, and means for impressing theresultant modulated wave trains upon a transmission medium.

2. An electrical Wave signal transmission system comprising anoscillator for generating carrier waves, a source of constantlyrepetitive electrical pulses, an intelligence wave source, means forapplying said pulses to said oscillator to cause it to generate shorttrains of carrier waves, the duration of a train after the end of theinitiating pulse depending on the duration of said pulse, means forangularly modulating the carrier waves comprising said Ywave trains inaccordance with the instantaneous amplitude of the intelligence wave,and means for impressing the resultant modulated wave trains upon atransmission medium.

3. An electrical wave signal transmission system comprising anoscillator including an electron tube having a cathode, an anode, acontrol grid, and regeneratively coupled anode and grid circuits, meansfor negatively biasing said grid .to

inhibit oscillation, a source of positive electrical pulses, means forapplying said pulses to said grid so as to cause said oscillator togenerate shorttrains of carrier waves, means to cause the time durationof said pulses to determine the time duration of said trains in excessof said pulses, an intelligence wave source, means for angularlymodulating said trains of carrier waves in accordance with theinstantaneous amplitude of the intelligence wave, and means forimpressing the resultant modulated wave trains upon a transmissionmedium.

4. An electrical wave signal transmission system comprising means forgenerating short trains of carrier waves, a source of constantlyrepetitive electrical pulses of constant time duration, an intelligencewave source, means kfor angularly modulating said carrier wavescomprising the respective trains in accordance with the instantaneousamplitude of said intelligence wave, a resonant circuit, means forapplying said carrier waves to said resonant circuit, means for applyingsaid pulses to said resonant circuit in such time relation with respectto said carrier waves that each pulse is applied substantially at theinstant when the carrier amplitude is passing through zero amplitude inthe same direction as the pulse, and means for coupling said resonantcircuit with a transmission medium.

5. The method of electrical carrier wave signal transmission whichcomprises generating short damped trains of carrier waves, angularlymodulating the carrier waves comprising the respective trains inaccordance with the rinstantaneous amplitude of an intelligence wave tobe transmitted, and amplitude modulating the initial amplitude ,of .therespective trains also in accordance with the instantaneous amplitude of4-saidintelligence Wave.

vto set said'oscillator into oscillation and are applied also to thesaid resonant circuit whilst the intelligence wave to be transmitted asthe angular modulation of said trains of waves is applied to control thefrequency of said resonant circuit.

7. An electrical signal transmission system as claimed in claim 4wherein the said intelligence wave is applied to control a capacity insaid resonant circ-uit.

8. An electrical signal transmission system as claimed in claim 4wherein said resonant circuit includes a capacity comprising thegrid-cathode capacity of a thermionic tube and means is provided toapply said intelligence wave to Vary the grid bias of saidlast-mentioned tube in accordance with the amplitude of saidintelligence wave.

9. An electrical signal transmission system as.

claimed in claim 4 wherein said resonant circuit is coupled to a radiantacting antenna.

10. The method of electrical carrier wave signal transmission whichcomprises generating a first series of vshort trains of damped carrierwaves, superposing a second series of short trains of damped carrierwaves on said first series, displaced a predetermined constant time withrespect to said first series, and varying the frequency of said carrierWaves of both said series in accordance with the instantaneous value ofan intelligence wave to be transmitted. l

11. An electrical wave signal transmission system comprising a resonantcircuit, means to vary the resonance of said circuit in accordance withthe instantaneous Values of an intelligence wave to be transmitted,means t0 apply an oscillation at the frequency of said resonant circuitto said circuit for repeated short periods of time, Whereby trains ofdamped oscillations are set up in said resonant circuit, means forrepeatedly shocking said resonant circuit at a constant time after thesetting up of said trains of oscillations, whereby additional trains ofoscillations of the same frequency are set up in said circ-uit having aphase relation to the oscillations of said first trains which isdependent on said constant time, a transmission medium, and means forimpressing the resultant modulated oscillation trains in said resonantcircuit upon said transmission medium.

12. An' electrical wave signal transmission system, comprising aresonant circuit, means to vary they resonance of said circuit inaccordance with an intelligence wave to be transmitted, means togenerate short trains of damped. carrier waves in said resonant circuitat the frequency of said circuit, means to generate additional trains ofcarrier waves in said circuit at a constant period of time after theinitiation of said rst wave trains, said period of time being such as togive a maximum output of said resonant circuit as a resultant of bothsaid trains of carrier Waves for one extreme frequency adjustment ofsaid resonant circuit, whereby the resultant of said two trains ofcarrier waves Will depend for its amplitude on said period of time, atransmitting medium, and

- means for impressing said resultant modulated repetitive electricalpulses of constant time duration, a resonant circuit, means to initiatein said resonant circuit trains of oscillations atthe irequency of saidcircuit, said means being operative for the time duration of saidpulses, whereby'the trains of carrier waves continue as dampedoscillations after the cessation of said initiating means, means toapply said pulses to said resonant circuit so as to create additionaltrains of

